The present invention relates to telecommunication systems. In particular, the invention concerns a method and system for signaling used in call setup.
A Centrex subscriber group is a subscriber group so implemented in a public exchange that, without being connected to a private automatic branch exchange, it can use functions typical of extensions connected to a PABX. A Centrex subscriber group is so implemented in the exchange that the exchange will provide a service resembling a PABX or a PABX network without employing separate branch exchanges. A feature typical of the functions of a Centrex exchange is a private numbering plan. It is a telecommunication service that allows the subscriber to use subscriber numbers deviating from the public numbering plan for communication between certain subscriptions within the area of one or more networks. The numbering plan can be implemented e.g. by reserving subscriber numbers beginning with a given sequence of digits for the use of an enterprise or a corresponding community.
A telephone exchange supporting the Centrex service is provided with a special database in which are stored all the numbers included in the public numbering plan that correspond to the private numbering plan. The Centrex service can be extended so as to allow it to be used between several telephone exchanges, in which case all the exchange have corresponding databases. In the signaling between two Centrex exchanges, a number included in a private numbering plan is converted into a number included in the public numbering plan. The subscriber, which may be e.g. an analog subscriber, an ISDN subscriber or a branch exchange, dials the numbers and the public network exchange performs the number conversion. If the subscriber dials a number included in a private numbering plan, then the public network exchange converts the number into a number consistent with the public numbering plan. In other words, the public telephone network uses numbers consistent with the public numbering plan, which are converted in Centrex exchanges so that the users of terminal equipment will see the numbers consistent with the private numbering plan. Therefore, for the operation of the Centrex system, a heavy database needs to be maintained, which means a waste of resources of the telecommunication system.
FIG. 1a is a diagrammatic representation of a prior-art system. Connected to a first exchange SW A is a Centrex subscription A. The Centrex subscription A implements a Centrex service that uses a private numbering plan in conjunction with the first exchange SW A. In the example, the Centrex service is implemented in the first exchange SW A, so the first exchange is a Centrex exchange. A corresponding service can also be implemented using an intelligent network.
A first telecommunication terminal TE1 is connected to the Centrex subscription A in the first exchange SW A using DSS1 signaling (DSS1, Digital Subscriber Signaling No. 1) or equivalent. The first exchange SW A is connected via a public telephone network ISDN to a second exchange SW B. For communication between the first and second exchanges, ISUP signaling (ISUP, ISDN User Part) is used.
In a corresponding manner, a second Centrex subscription B is connected to the second exchange SW B, which likewise is a Centrex exchange. Connected to the Centrex subscription B in the second exchange SW B is a second telecommunication terminal TE2 using DSS1 signaling. In the example, the subscriber lines of both terminals TE1, TE2 are comprised in a private numbering plan implemented via a Centrex service. The first and second telecommunication terminals TE1, TE2 may be ISDN subscribers, analog subscribers or branch exchanges.
In this prior-art solution, the first exchange SW A comprises a Centrex subscriber database, which contains all A-subscriber and B-subscriber numbers both in a long form and in a short form, i.e. in forms consistent with the public and the private numbering plans. Thus, e.g. a short B-subscriber number 123 corresponds to a long number 456 789. The first exchange SW A and the second exchange SW B have identical databases. When an A-subscriber calls a B-subscriber by dialing the number 123, this number is converted into 456 789 and the call is routed using the long number in the public telephone network ISDN.
When the second exchange B receives an A-subscriber number, e.g. 456 789, it checks the number received against the numbers in the Centrex database. If a corresponding number is found in the database, then the long number 456 789 is again converted into the short number 123, which is transmitted to the display 10 of the second terminal TE2, and the second terminal TE2 generates a ringing tone as used for internal calls. Thus, the user of the second terminal TE2 can recognize the incoming call as an internal call.
A private numbering plan and/or special group functions can also be implemented in a virtual private network VPN which uses signaling consistent with the QSIG standard. The virtual private network VPN is gaining ground in network solutions in which an economic network protected against outsiders is needed. QSIG is an ISDN common channel signaling system designed for use in private networks. The QSIG signaling was developed by an open group of enterprises including all of the largest European PABX manufacturers. The designation QSIG refers to signaling transmitted over an ISDN Q-point. QSIG defines a number of protocols which can be used for interaction between a plurality of PINX""s (PINX, Private Integrated services Network eXchange). A PINX is connected to the nearest telephone exchange e.g. via a 2 Mbit/s link carrying QSIG signaling. A QSIG protocol largely resembles the ITU-T (International Telecommunication Union) Q.931 protocol. The essential difference between these is that Q.931 is an asymmetric protocol while QSIG is symmetric.
A virtual private network can also be utilized in an IP environment (IP, Internet Protocol), in which case it is possible to implement e.g. remote operation of a protected intranet used by an organization, i.e. an extranet. Extranet means a data network which is implemented between organizations using the Internet as a data transmission means and which can be used to interconnect the intranets of the organizations included in the system.
Using a virtual private network, it is possible to interconnect Centrex subscriber groups, Public Branch Exchanges (PBX) and individual subscribers. A VPN system can also be implemented using a public telephone network for data transmission. In the VPN system, a private numbering plan is identified by a special Corporate Network Identifier CNID. Subscriber identification is performed using a CNID parameter packed in the APP parameter (APP, Application Transport Parameter). The APP parameter additionally contains the A-subscriber and B-subscriber numbers in the private numbering plan, e.g. in the internal numbering used in an enterprise. However, matching VPN and Centrex systems is a problematic task because the prior art Centrex system does not understand the APP parameter used in QSIG signaling.
The object of the invention is to eliminate the problems referred to above. A specific object of the invention is to disclose a new type of method and system for signaling used for call setup in a situation where at least one of the telecommunication terminals belongs to a Centrex subscriber group. The present invention enables cooperation between a QSIG VPN network and a Centrex subscriber group in a manner transparent to the user, i.e. so that the user will perceive it as consistent cooperation regardless of the system to which the terminal equipment is connected.
The basic idea of the invention is to use in the Centrex exchange the same APP parameter as is used by a QSIG VPN network in VPN ISUP signaling, both for communication with another Centrex exchange and for communication with a VPN exchange.
The invention concerns a method for signaling used for call setup in a telecommunication system. The telecommunication system comprises a telephone network, a private numbering plan implemented in the telephone network and a Centrex exchange which belongs to the telephone network and implements the private numbering plan. The system further comprises a first telecommunication terminal, which is connected to the Centrex exchange via a Centrex subscription, and a second telecommunication terminal, which is connected to the telephone network with a number included in the private numbering plan. The second terminal is either a VPN subscriber or a Centrex subscriber. In the method, a connection is set up between the first and the second terminals by using the number included in the private numbering plan. According to the invention, when a connection is being set up from the first terminal to the second terminal, a parameter comprising an identifier corresponding to the private numbering plan as well as the identifiers of the calling and called subscribers in the private numbering plan is generated for the connection in the Centrex exchange. In addition, when a connection is being set up from the second terminal to the first terminal, the subscription of the second terminal is identified in the Centrex subscriber data by the said parameter.
In an embodiment of the invention, the said parameter is a CNID parameter consistent with the QSIG standard. The CNID parameter, which is included in the APP parameter, is stored in the subscriber data for the Centrex subscription in the public network subscriber database. When the Centrex subscriber sets up a call, the CNID parameter is retrieved from the subscriber database of the exchange and sent further in the network inside the APP parameter. In another application, this parameter is a short number consistent with the QSIG standard and is included in the APP parameter.
The APP parameter is preferably sent in an ISUP message during call setup. In an embodiment, the Centrex subscription is implemented as a QSIG-Virtual-Private-Network compatible subscription. In this case, the Centrex subscription is compatible with the QSIG system in respect of certain services. The value of the CNID parameter comprised in the APP parameter is stored in the Centrex exchange.
The second telecommunication terminal can be connected to an exchange or subscription that implements a corresponding functionality; in an embodiment, the second terminal is connected to an exchange implementing a QSIG-Virtual-Private-Network functionality. In another embodiment, the second terminal is connected to another Centrex subscription according to the present invention.
The invention also concerns a system for signaling used in call setup in a telecommunication system as described above. In the system of the invention, the Centrex exchange comprises means for generating for a connection being set up from a first telecommunication terminal to a second telecommunication terminal a parameter comprising an identifier corresponding to a private numbering plan as well as A-subscriber and B-subscriber identifiers in the private numbering plan. In addition, the Centrex exchange comprises means for identifying the subscription of the second terminal by the said parameter, which preferably is a CNID parameter consistent with the QSIG standard, when a connection is being set up from the second terminal to the first terminal. In another embodiment, the subscription is identified by a short number consistent with the QSIG standard and included in the APP parameter.
In an embodiment, the Centrex exchange comprises means for sending the APP parameter in an ISUP message when a connection is being set up. The Centrex subscription is preferably implemented as a QSIG-Virtual-Private-Network compatible subscription. In an embodiment, the Centrex exchange comprises a database for the storage of the CNID parameter comprised in the APP parameter. In practice, in an embodiment, the second terminal used is connected to a subscriber line implementing QSIG-Virtual-Private-Network functionality, in another embodiment it is connected to a Centrex subscriber line.
As compared with prior-art solutions, the invention has the advantage of obviating the need to provide in the Centrex exchange a resource-consuming database for each subscriber, such databases being difficult to maintain. Moreover, the invention enables a combination of Centrex and VPN services, thus allowing a private numbering plan and special group services to be used globally regardless of locally implemented exchange systems. Another solution for cooperation between a Centrex subscription and a VPN subscription would be to implement a Centrex subscriber database in the VPN exchange, which would enable the VPN exchange to function as a Centrex exchange. The solution of the present invention is better, cheaper and simpler than the prior-art Centrex system as it obviates the need to maintain heavy databases. Besides, the invention allows a more versatile operation of the exchange system; the QSIG emulation referred to allows the above-described Centrex VPN subscriber to act either as a Centrex subscriber or as a QSIG VPN subscriber. In addition, the subscriber is able to act in accordance with either type of functionality as required by the call in each case, which means that cooperation between Centrex and VPN systems is further improved.